1. Field of the Invention
The present invention relates to a method for detecting the capacity of a battery loaded on a piece of electronic equipment, such as a portable personal computer, for furnishing the power. The present invention also relates to an associated battery pack and to an associated electronic equipment system.
2. Description of the Prior Art
In order to know the capacity (residual capacity) of a battery, such as a lithium ion battery, it is generally practiced to estimate the residual capacity from a terminal voltage of the battery or to integrate the efflux current to estimate the residual capacity.
As a method for detecting the capacity (residual capacity) of such battery, there are currently proposed a voltage method and a current integrating method. The voltage method measures the voltage of the battery to calculate the capacity of the battery based on the correlation between the voltage and the capacity of the battery. The current integrating method integrates the current of the battery with respect to time to calculate the capacity of the battery. In the voltage method, the capacity calculation accuracy is high when the current of the battery is small, conversely, in the current integrating method (coulomb method), the capacity calculation accuracy is high when the current of the battery is large. Since the voltage method is based on the voltage, there is no integrating error, but there is a direct error. Since the current integrating method integrates and updates the current with respect to a reference value, the integration error is significant, though the direct error is small.
The voltage method calculates the capacity from the correlation between the capacity and the terminal-to-terminal voltage of the battery (cell voltage). Since the battery (cell) has an internal resistance and a hence A the terminal voltage is fluctuated depending on the flowing current, correction is applied based on the current multiplied by the inner battery voltage. If the current is enlarged, the amount of correction becomes larger to increase the error.
The current integrating method integrates the current with respect to time to find the electrical quantity Ah. For improving the precision, current measurement accuracy needs to be raised. For this reason, an operational amplifier or an analog-to-digital (A/D) converter of extremely high precision is used. Nevertheless, there is produced significant error if charging/discharging is repeated over an extended time interval, or if the current is of a small magnitude, so that contrivances such as calibration are occasionally required such as at the time of charging the battery to its fill capacity. If the calibration timing is lost, significant errors are inevitably produced.